This invention relates to improved processes and products for effecting laser-induced thermal transfer imaging.
Laser-induced thermal transfer processes are well-known in applications such as color proofing and lithography. Such laser-induced processes include, for example, dye sublimation, dye transfer, melt transfer, and ablative material transfer. These processes have been described in, for example, Baldock, U.K. Pat. No. 2,083,726; DeBoer, U.S. Pat. No. 4,942,141; Kellogg, U.S. Pat. No. 5,019,549; Evans, U.S. Pat. No. 4,948,776; Foley et al., U.S. Pat. No. 5,156,938; Ellis et al., U.S. Pat. No. 5,171,650; and Koshizuka et al., U.S. Pat. No. 4,643,917.
Laser-induced processes use a laserable assemblage comprising (a) a donor element that contains a thermally imageable layer, the exposed areas of which are transferred, and (b) a receiver element that are in contact. The laserable assemblage is imagewise exposed by a laser, usually an infrared laser, resulting in transfer of exposed areas of the thermally imageable layer, also referred to as material, from the donor element to the receiver element. The (imagewise) exposure takes place only in a small, selected region of the laserable assemblage at one time, so that transfer of material from the donor element to the receiver element can be built up one pixel at a time. Computer control produces transfer with high resolution and at high speed. The laserable assemblage, upon imagewise exposure to a laser as described supra, is henceforth termed an imaged laserable assemblage.
Laser-induced processes are generally faster than analog processes and may result in transfer of material with high resolution. However, such laser imaging processes that image to a temporary receiver not only have defects and limitations associated with the imaging process but also in the processes and materials used to transfer images from the temporary image receiver to the final substrate. One such defect that occurs is dot movement where a row of dots (or some individual dots within the row) move during the imaging or image transfer processes resulting in a visible white area in the colored image or a distortion in color and resolution. Another potential imaging defect is xe2x80x9cbandingxe2x80x9d, which can be associated to a write engine with an external drum configuration. xe2x80x9cBandingxe2x80x9d can be defined as a defect where the thermally transferred material deposited on the temporary receiver in one revolution of the external drum write engine does not exactly meet (butt) the thermally transferred material deposited in the subsequent drum rotation. The result is a xe2x80x9cwhite linexe2x80x9d in the image area of some dimension. The xe2x80x9cbandingxe2x80x9d defect can be reduced by xe2x80x9cover-writing or overlappingxe2x80x9d the deposited material in the previous drum rotation with the deposited polymer material in the successive drum rotation (as opposed to xe2x80x9cbuttingxe2x80x9d the two deposited materials in 2 drum rotations). The slight xe2x80x9coverwrite or overlapxe2x80x9d of the two deposited materials reduces or eliminates the xe2x80x9cspacexe2x80x9d between the deposited polymer materials in two successive drum rotations. Another problematic defect associated with both the imaging and subsequent image transfer steps can be called swath boundary cracking. This defect manifests itself where a non-uniform deposition of material occurs between successive drum revolutions. Compared to the xe2x80x9cbandingxe2x80x9d defect where a space occurs between the deposited materials of two drum revolutions, swath boundary cracking has a very small amount of material deposited in the xe2x80x9cspacexe2x80x9d and when the image is transferred to the final substrate by high temperature and pressure laminationxe2x80x94a crack occurs in the xe2x80x9cboundaryxe2x80x9d area between the two successively deposited materials.
Another limitation of current processes involving the physical transfer of images from a temporary image receiver to the final substrate is the lamination conditions. The image built up on the temporary image receiver sheet must be transferred to the final or permanent substrate (usually paper) via an acceptable lamination process. The lamination conditions must be controlled in a narrow range, and even so, the lamination conditions limit image quality, productivity, process latitude and the type of permanent substrate used.
Furthermore, another severe limitation present in the use of NIR (near infrared) dyes in the thermally imageable layer or colored layer of the donor or thermally imageable element is that color purity is sacrificed in the transferred image. The NIR dye imparts undesired color in the image areas.
Consequently, a need exists for a process for providing color images, which operates effectively at the highest speeds possible (greater productivity) and lamination latitude, and which affords the highest achievable image quality on receiver elements upon thermal imaging and subsequent image transfer processes to the permanent substrate. A need also exists for a process that provides flexibility in the type of permanent substrates that may be used. A need also exists for a process that provides a vehicle or mechanism for eliminating the color impurities associated with the use of NIR dyes in the system. To date, most NIR dyes absorbing in the actinic region of diode lasers (830 nm) have long absorbing xe2x80x9ctailsxe2x80x9d into the visible spectrum (400-700 nm) which give rise to significant undesired color in image areas. A need further exists for new assemblages, image proofing systems, printed proofs and similar products which overcome the shortcomings described above.
Improved processes and products for laser induced thermal imaging are disclosed herein.
In a first embodiment, this invention provides a method for making a color image comprising:
(1) imagewise exposing to laser radiation a laserable assemblage comprising:
(A) the thermally imageable element comprising a thermally imageable layer; and
(B) a receiver element in contact with the thermally imageable layer of the thermally imageable element; the receiver element comprising:
(a) an image receiving layer; and
(b) an receiver support; whereby the exposed areas of the thermally imageable layer are transferred to the receiver element to form a colored image on the image receiving layer;
(2) separating the thermally imageable element (A) from the receiver element (B), thereby revealing the colored image on the image receiving layer of the receiver element;
(3) contacting the colored image on the image receiving layer of the receiver element with an image rigidification element comprising:
(a) a support having a release surface, and
(b) a thermoplastic polymer layer, the colored image being adjacent the thermoplastic polymer layer during said contacting, whereby the color image is encased between the thermoplastic polymer layer and the image receiving layer of the receiving element;
(4) removing the support having a release surface thereby revealing the thermoplastic polymer layer; and
(5) contacting the revealed thermoplastic polymer layer from step (4) with a permanent substrate.
Preferably, the thermally imageable element is formed by applying a thermally imageable layer comprising a colorant to a base element.
In a separate embodiment, this invention provides a method for making a color image, further comprising after step (5):
(6) removing the receiver support.
In another embodiment, the invention provides a method of bleaching a polymethine type NIR dye contained in a laserable assemblage which comprises contacting the dye in the laserable assemblage with a oxidant type bleaching agent selected from the group consisting of hydrogen peroxide, organic peroxides, hexaaryl biimidazoles, halogenated organic compounds, persulfates, perborates, perphosphates, hypochlorites and azo compounds; whereby the NIR dye is bleached by the bleaching agent.
In still another embodiment, the invention provides an image proofing system comprising:
(a) a laser generated halftone dot color thermal image formed on a crystalline polymer layer, the crystalline polymer layer being located on a first temporary carrier; and
(b) a thermoplastic polymer layer laminated to the crystalline polymer layer whereby the color image is encased between the crystalline polymer layer and the thermoplastic polymer layer, the thermoplastic polymer layer being located on a second temporary carrier.
In still another embodiment, the invention provides a printed proof comprising:
(a) a laser generated halftone dot color thermal image formed on a crystalline polymer layer; and
(b) a thermoplastic polymer layer laminated on one surface to the crystalline polymer layer and on the other surface to a final receptor, whereby the color image is encased between the crystalline polymer layer and the thermoplastic polymer layer.